Recent legislative developments now require on-board monitoring of a vehicle exhaust catalytic convertor efficiency as part of the vehicle's air/fuel and emission control operation. Such monitoring is typically accomplished by placing an exhaust gas oxygen (EGO) sensor before and after the catalyst. The outputs EGO sensors are processed to quantify exhaust gas perturbations caused by rich-lean air/fuel mixture switches during operation of the engine. Examples of prior monitoring arrangements are disclosed in U.S. Pat. No. 5,357,751 to Orzel, which shows monitoring of a single inlet/outlet type catalytic convertor, and U.S. Pat. No. 5,385,016 to Zimlich et al. which shows monitoring of a Y-pipe exhaust system having two upstream EGO sensors and one downstream EGO sensor.
Of particular concern to the present invention is a monitoring arrangement for Y-pipe exhaust systems. Generally, the arrangement taught in U.S. Pat. No. 5,385,016 is able to provide monitoring without placing a third upstream EGO sensor at the Y-pipe junction, i.e., the exhaust bank blending location, by using the two upstream EGO sensor outputs to infer what the likely blended EGO sensor output would be.
One potential source of error in such an inferred EGO arrangement relates to the phase coherency of the two independent fuel control banks on the engine. More specifically, as one bank falls out of phase with the other, rich-lean exhaust gas pulsations cancel each other out. In a symmetrical y-pipe exhaust system (i.e., the exhaust paths from separate exhaust manifolds to the Y-pipe junction are the same length), the phasing of the banks can be used to control switching of the selected inferred EGO signal.
However, in an asymmetrical exhaust system configuration, the different exhaust path lengths cause the banks to fall in and out of phase without regard to upstream phase coherency. As a result, a need exists for a way of ensuring accuracy in an inferred EGO sensor signal for asymmetrical Y-pipe exhaust systems.